realpath libc defined but not implemented (IDFGH-6119)

[ivmarkov]

Currently, the Rust STD equivalent of realpath (fs::canonicalize) is implemented to call into realpath, and as a result, usage of fs::realpath results in a linkage error. This is not so ideal.

How do we feel about implementing realpath in ESP-IDF itself? Basically "Option A" as documented here:
https://github.com/ivmarkov/rust-esp32-std-demo/blob/6611e765441d8cc3a6d018ece935f521b4fe7e79/src/main.rs#L975

[igrr]

@ivmarkov Thanks for the suggestion, we can implement realpath in IDF. We can't use the newlib code you've linked because it is released under LGPL, i think. However given the absence of symlinks the implementation shouldn't be difficult.

[ivmarkov]

Sure - the linked code was just an example that the function can be trivially implemented in terms of parsing the path and handling the ..s and skipping the .s (that is - in the absence of symlinks on FAT and no current directory).

If you decide to implement this one, perhaps it makes sense to also implement:

• getcwd - to always return /, as realpath would be expanding relative directories as if cwd is /
• chdir - to fail with ENOSYS

[ivmarkov]

@igrr realpath implementation works great. Tested:

• Usage of realpath via Rust's fs::canonicalize API
• Crate panic_abort now works without the panic_immediate_abort option, which allows Rust code to print nice panic / assertion messages before giving control to ESP-IDF's panic handler.

Please merge into v4.4
I'll probably backport this code into v4.3.1 in the form of a patch that is applied (as usu.) when compiling the ESP-IDF for Rust.

[igrr]

Backported to release/v4.4 in f158a0d, will be part of v4.4-rc1.